He was commissioned as cornet in the Worcester Fencible Cavalry in 1798, was promoted to lieutenant in September 1799, and served in Ireland until the regiment was disbanded in 1800.

It played the principal part in the Battle of Sahagún on 21 December 1808, and to cover the retreat of General Sir John Moore's army to Corunna.

At the passage of the river Esla on 31 May 1813, Thackwell commanded the leading squadron which surprised a French cavalry picket and took thirty prisoners.

On 1 March 1814, after passing the river Adour, Thackwell was in command of the leading squadron of his regiment, and had a creditable encounter with the French light cavalry, on account of which he was recommended (unsuccessfully) for a brevet majority by Sir Stapleton Cotton.

[5] Thackwell became a colonel in the army in January 1837, and in May 1837, by exchange, he took command of the 3rd The King's Own Dragoons, travelling with his new regiment to India and arriving in Calcutta in November 1837.

He was present at the Siege of Ghazni, and he commanded the second column of the part of the army which returned to India from Kabul in the autumn of 1839.

To enable his own army to follow them, Gough sent a force of about eight thousand men under Thackwell to pass the river higher up, and help to dislodge the Sikhs from their position by moving on their left flank and rear.

Thackwell found the nearer fords impracticable, but crossed at Wazirabad, and encamped on the morning of 3 December near Sadulapur.

They kept up a heavy fire of artillery until sunset, and attempted to turn the British flanks, but there was very little fighting at close quarters.

In the course of the afternoon Thackwell received authority to attack if he thought proper; but as the enemy was strongly posted, he deemed it safer to wait till next morning.

In his despatch of 26 February 1849 Gough said: ‘I am also greatly indebted to this tried and gallant officer for his valuable assistance and untiring exertions throughout the present and previous operations as second in command with this force.’ Thackwell received the thanks of parliament for the third time, and was advanced to GCB on 5 June 1849.

His second son, Major-General William de Wilton Roche Thackwell (1834–1910), served in the Crimean War and in Egypt in 1882.

His fourth son, Francis John Roche Thackwell, served in the Royal Irish Lancers, and died in India in 1869 from wounds inflicted by a tiger.